1. The recent growth of new people that post once then leave.
There has been a large increase in these people for some reason. Whatever the case may be. Not as many people are staying for long periods.

2. The lack of a common cause.
This forum seems to have almost always had someone to go against. Some kind of fundamentalist or similar nutjob. Someone who was so outrageously stupid that people couldn't help but yell at.

3. Lack of advertising.
Let's face it. The Thinking Atheist has not hit any new demographics for awhile now. Not as many new eyes see this forum on a daily basis than there used to be.

4. Atheism right now.
There's not all that much going on in the atheist world that is relatively new. Nothing that sparks the fires in peoples hearts to blog and discuss. Yes yes yes, we're making good strides....but it's all been fairly dull.

It's far from dead - there are a lot of others that are really much deader. (dead, deader, the deadest? :shocking

But, the impossibility of being deader than dead aside, every forum I have ever been a contributing member of has gone quieter during the summer months. And especially during school vacations. People spend time outside and with their families. They don't go online and try to find new places to hang out. They will be doing that in the fall...

Science is the process we've designed to be responsible for generating our best guess as to what the fuck is going on. Girly Man

On every forum I have been on, the number of posts/traffic slows down during the summer months. As someone said before, people are outside more, going on vacations, etc. during this time of the year. They are on the computer less so things slow down.